Project Summary/Abstract: Lymphedema is the most common lymphatic anomaly and it is responsible for considerable morbidity, with no current effective treatments. Improper drainage of extravasated protein-rich fluid from the tissues causes it to accumulate, resulting in lymphedema. This condition frequently involves defective lymphatic valve development, yet the epigenetic modifiers underlying lymphatic-valve morphogenesis remain unknown. Two opposing classes of histone-modifying enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs) regulate the acetylation state of histones. Acetylation by HATs is generally associated with transcriptional activation while HDAC-mediated deacetylation usually results in transcriptional repression. HDACs lack intrinsic DNA-binding domains but modify epigenome in a signal-dependent manner via their interactions with chromatin modifiers, transcription factors, and cofactors. Our findings suggest a novel chromatin dependent, but deacetylase-independent role of Hdac3 in murine lymphatic valve development. We find that Hdac3 functions as an essential flow-responsive epigenetic switch to establish a specific transcriptional program for lymphatic valve development. Our data challenge long-held assumptions that HDACs replace HATs to promote both histone deacetylation and repression of transcription. The goal of this research program is to identify how Hdac3 establishes and maintains a specific transcriptional program for lymphatic valve development. In addition, proposed studies will identify the mechanisms by which different transcription factors, kinases, and phosphatases involved in signaling pathways regulate the chromatin recruitment, phosphorylation, and function of Hdac3 within developing lymphatic vasculature. Despite intense study in the area of epigenetics, very little is known about the role of epigenetic and chromatin modifiers in the field of lymphatic development. The set of experiments outlined in this proposal have broad significance not only for understanding how signaling pathways intersect with chromatin modifiers to regulate lymphatic valve development, but also could be highly applicable to the entire field of congenital cardiovascular diseases.